Hello guys,

Which topology/converter should we use to have buck converter with an additional boost option? Mean, I will have cascaded step down (buck converter 48 V ~14V ~3.3V) .I will generally supply 3.3 uCs but sometimes, i will need to feed 5V sensors and I need to boost my output from 3.3V to 5V. What do you suggest me to use ? I could not see in the internetPeople I really need help..

 

Well if your stepping down then why not step down from 48v to 14v to 5v to 3.3v ???

 

What is the 14V for? If you are buck converting 48->14->3.3 then it would be cheaper and more efficient to just go 48->3.3V

If I needed 3.3V and 5V I would make my buck converter go 48->5 and then drop a linear regulator on it for the 3.3V. If you insist on using one buck for both 3.3 and 5V it's not possible (you just adjust the voltage sense resistors or the reference voltage) but that leaves you the problem your 3.3V just became 5V!

Not insurmountable, you follow the 48->3.3V(sometimes 5) buck with a low dropout 3.3V regulator, which gives you about the same 3.3V all the time: just not as solid a 3.3V as a 5V->3.3V linear reg.

 

Well if your stepping down then why not step down from 48v to 14v to 5v to 3.3v ???

Yes, you are right, but I heard about cascaded buck-boost converter and normal buck--boost converters, why do they used to utilize from these converters in the market if we can directly use many cascaded buck operation?

What is the 14V for? If you are buck converting 48->14->3.3 then it would be cheaper and more efficient to just go 48->3.3V

If I needed 3.3V and 5V I would make my buck converter go 48->5 and then drop a linear regulator on it for the 3.3V. If you insist on using one buck for both 3.3 and 5V it's not possible (you just adjust the voltage sense resistors or the reference voltage) but that leaves you the problem your 3.3V just became 5V!

Not insurmountable, you follow the 48->3.3V(sometimes 5) buck with a low dropout 3.3V regulator, which gives you about the same 3.3V all the time: just not as solid a 3.3V as a 5V->3.3V linear reg.

Erniem, do you think that will work to go from 48V to 5V with buck?
Duty cycle is too small around 10% and I will need to switch really fast which brings EMI,EMC problems. Let's say If u design that buck converter, VL= Vin-Vout =Ldi/dt => 43= L* di/D*T => let's assume we have ripple 15mA and freq 150kHz so 43 = L*15*10^-6 / (10*150*10^3)
therefor, L ~3mH which is really big and will cost a bit expensive and I have space and price pressure on me Maybe I am wrong but I am calculating these values.